This invention relates to ferroresonant voltage regulating circuits and in particular to a DC power supply which uses a ferroresonant power transformer to achieve voltage regulation, and which incorporates a stabilizer circuit to eliminate instabilities in the power supply under certain common operating conditions.
Ferroresonant regulators presently find widespread use in the power supply field. Ferroresonant devices utilize transformer saturation to obtain regulation over line voltage changes. Secondary saturation insures that the secondary voltage cannot increase beyond a certain value, independent of variations in primary (input) voltage.
Among the many advantages of ferroresonant power supplies the most important probably is their excellent voltage regulation during static and dynamic line voltage changes. In addition, ferroresonant power supplies are reliable, of relatively low cost, simple in structure and of small size. They have inherent short circuit protection, good efficiency and a high input power factor.
In operation when the AC input voltage to the ferroresonant transformer is high enough the transformer core under the secondary winding saturates at a point in each AC half-cycle. Further increases in line voltage beyond the saturation point are absorbed by primary inductance. Therefore, the secondary voltage remains constant over changes in line voltage. A more detailed description of ferroresonance and its application to regulated power supplies can be found in Transformer and Inductor Handbook, William T. McLyman, Marcel Dekker, Inc. (1978), which is incorporated by reference, as if fully set forth herein.
Traditional ferroresonant power supplies use a high power bleeder resistor to dampen oscillatory tendencies of the power supply at a light or no load condition. Without the bleeder resistor, occurrences such as line interrupts, transients or abrupt removal of the output load can easily send the ferroresonant power supply into unstable oscillation. In fact, it is not uncommon for a ferroresonant power supply to enter an unstable oscillatory mode in a no load condition, even in the absence of a line transient or interrupt. When a ferroresonant power supply goes into an unstable oscillatory condition, the DC output voltage of the supply can easily reach three to five times its normal magnitude. Such levels of output voltage can damage circuits that depend on a regulated voltage from the ferroresonant power supply.
In order to provide sufficient damping to prevent the ferroresonant power supply from entering an unstable oscillatory state, a minimum load current of about 5% to 10% of the maximum rated load current must be maintained. To accomplish this, the prior art has traditionally used a large bleeder resistor to guarantee such a minimum current output under no load conditions. However, the bleeder resistor used in the prior art dissipates power continuously even at full load where it is unnecessary. The presence of a bleeder resistor, which consumes up to 10% of the available output power from the ferroresonant power supply, creates significant heat in the area of the ferroresonant power supply in high power applications. The generated heat may be sufficient to require a fan or heat sink to dissipate the heat away from the regulator, or it may require oversized power components.
Operation of a ferroresonant power supply under such conditions is undesirable since the energy loss in the bleeder resistor required for stability purposes represents a reduced efficiency of the power supply which is otherwise of high efficiency. In addition, equipment such as a fan or heat sink required to dissipate the heat generated by the bleeder resistor add cost to an otherwise relatively inexpensive ferroresonant power supply. High efficiency and low cost are two of the most desirable features of a ferroresonant power supply. Therefore, there is a need for a ferroresonant power supply which can operate stably over a no load to full load range without requiring the continuous dissipation of a portion of the total regulator output into a bleeder resistor.
An object of this invention is to provide a new and improved construction of a ferroresonant power supply which maintains operational stability over input line transients and rapid variations in output load without the need of a continuous minimum power dissipation.
A further object of this invention is to provide a stabilizer circuit for a ferroresonant power supply which minimizes the need for a bleeder resistor to stabilize the power supply output.